The choice between a solid and a hollow block is usually made on price, and usually made wrongly. The two units answer different structural questions, and the cheaper one on the invoice is often the more expensive one on the programme.
A solid block carries load. Where a wall is doing structural work — a load-bearing external wall, a compound wall taking wind, a retaining course below plinth — the density that makes the unit heavy is the same density that makes it strong. IS 2185 (Part 1) covers both, but the grades diverge sharply: solid units run 5.0 to 8.5 N/mm², hollow units 3.5 to 5.0.
A hollow block buys back weight. In a framed structure, where the masonry is infill rather than support, the cavity earns its place three times over: less dead load on the slab, fewer units per square metre of wall, and a mason who can lay a course without a second pair of hands. On a tall building the saved dead load compounds down the frame into smaller sections and less steel.
The failure we see most often is a hollow block used below plinth or in a retaining course, where it takes water and lateral pressure it was never graded for. If the wall is holding anything back, specify solid.